For every +, the maximum HP and MP increase by 1%.From +3 to +5, the devil skill will change and give 25% damage increase as well as 15% speed increase.Also, mob defense ignore stat with 1% probability is added as a green (yes, green and not blue like on other equipment).The skill icon will change a little, but the character's look when using stays the same.You get an additional buff skill right to the devil skill which gives 300 parry ratio increase.From +6 on, the damage increase is 30%, the speed increase is 20%.The mob defense ignore probability will raise to 2%. The parry ratio buff doesn't change, but the devil skill icon changes again and the character will look like this when using it:

More screenshots from cSRO site:

+5 devil spirit skill

+6 devil spirit with green mob defense ignore

Parry ratio increase buff

I guess it will still take some time until the update comes to iSRO, but you should look forward to it!